The present invention relates to the technological field of gas turbines. It relates to a heat shield for a gas turbine, which heat shield encloses in particular the rotating blades of a stage of the gas turbine in an annular manner and consists of a plurality of heat-shield segments, which are arranged one behind the other in the circumferential direction, are curved in a circular-segment shape and are cooled from outside, and the longitudinal sides of which are designed as correspondingly curved rails running in the circumferential direction and having in each case a first arm projecting in the axial direction, and which, for fastening to the casing of the gas turbine, are mounted with the first arms in each case in an annular intermediate space of constant width.
2. Description of the Related Art
A heat shield in a gas turbine generally has the function of limiting the flow space for the hot gas and of protecting the casings and supporting structures of the gas turbine from the hot gas and an excessive heat input. To this end, it is necessary to cool the heat shield, as described, for example, in U.S. Pat. No. 3,864,056. The cooling, which is normally achieved by the flow of cooling air against the heat shield on the side remote from the hot gas, causes temperature differences in the heat shield and thus internal thermal stresses, which may subject the heat shield to excessive loading.
On account of the cooling, a temperature gradient results between the front side facing the hot gas and the heat-shield rear side to which cooling air is admitted. Consequently, the individual segments of the heat shield, which are curved in a circular-segment shape on account of the rotational symmetry of the gas turbine, attempt to stretch or xe2x80x9cstraightenxe2x80x9d. In order to limit the effect of the internal stresses resulting therefrom, it has been proposed, for example, in the publication U.S. Pat. No. 5,071,313 mentioned at the beginning, to make T-shaped slots in the laterally arranged fastening rails of the segments.
Another problem results from the thermally induced straightening or bending of the segments in connection with their mounting or fastening on the casing of the gas turbine. If the heat shield or its segments are prevented from straightening by the type of mounting, internal stresses build up, and these internal stresses may exceed the stability limits of the material used and may cause cracks. Experience with heat shields in various gas turbines has shown that the causes of cracks have often been internal stresses which have been caused by temperature differences. In order to avoid such a build-up of stresses, it has been proposed in EP-B1-0 728 258 to mount the segments of the heat shield on the casing by means of integrally formed hooks, first hooks being suspended with little play in the center of the segment, whereas second hooks are suspended at the ends of the segment with so much play that no mechanical stresses are produced by the suspension during the thermally induced stretching of the segments and the associated displacement of the second hooks.
However, the known mounting, subject to play, by means of the first and second hooks has various disadvantages: since the central fixing of the segments by the first hooks is not sufficient in order to fix the segments in their position as an entity, a band (62 in FIG. 2 of EP-B1-0 728 258) must additionally be inserted in order to hold the segments in a stable position. This requires considerable additional outlay during manufacture and installation of the heat shield.
The object of the invention is therefore to provide a (segmented) heat shield for a gas turbine, which heat shield avoids the disadvantages cited and in particular permits robust fastening of the segments in a simple construction and with ease of installation of the segments without substantially impairing the thermally induced straightening of the segments.
The essence of the invention consists in mounting the individual segments of the heat shield virtually free of play at their respective ends via rails which are continuous at the longitudinal sides, whereas sufficient play is provided in the center region, so that the center region can be displaced in the radial direction without impairment during the thermally induced straightening of the segments. In this case, the bearing points of the segments at the segment ends form pivot points for the stretching or straightening.
A first preferred embodiment of the heat shield according to the invention is characterized in that the intermediate spaces- in each case have an outer radius of curvature and an inner radius of curvature, in that the first arms, in the cold state of the heat-shield segment, in each case have an outer radius of curvature and an inner radius of curvature, and in that the two outer radii of curvature are essentially equal and the inner radius of curvature of the first arms is smaller than the inner radius of curvature of the intermediate spaces. Since in this embodiment the requisite play in the center region of the segments is produced by different radii of curvature, the segments can be machined to different radii of curvature in a simple manner with a grinding wheel. At the same time, steps or discontinuities in the marginal region of the segments at which unfavorable mechanical stress states may occur are avoided.
A preferred development of this embodiment is distinguished by the fact that the inner radius of curvature of the first arms is selected in such a way that, due to the thermally induced straightening of the heat-shield segment during heating to the operating temperature of the gas turbine, it changes into a radius of curvature which is approximately equal to the inner radius of curvature of the intermediate spaces. This advantageously achieves the effect that, in the cold state, the segment bears against the fastening with the entire outside of the first arms, whereas, in operation, i.e. after the thermally induced straightening, it bears against the fastening with the entire inside, so that all the forces between the segment and the fastening can always be transmitted over a large area.